Conventionally, a liquid crystal display (hereinafter, referred to as LCD) has been used as a display apparatus for a word processor or a personal computer. Due to its capability of easy miniaturization and its advantages of being thin, lightweight, and the like, the LCD has been more and more frequently used in these days, for example, as a display of a portable telephone and the like.
As a type of the LCDs, there exists a passive matrix type LCD for driving so-called Twisted Nematic type (TN type) liquid crystal display device and Super Twisted Nematic type (STN type) liquid crystal display device without using any thin-film transistor. Besides a conventional line sequential scanning system (duty system) such as an APT (Alt Pleshko Technique) drive system, or an IAPT (Improved APT) system, which is obtained by improving the APT system, various drive systems are conceived as systems for driving these LCDs.
In contrast with such a conventional line sequential scanning system, a multiline addressing drive system (MLA drive system), which is a multiline simultaneous drive system for simultaneously selecting and driving a plurality of scanning lines, has also been proposed.
For example, JP 6-27904 A discloses an example of the MLA drive system, called a Multi-Line Selection (MLS) drive system. More specifically, this drive system is for selecting L row electrodes at a time. A selection voltage for the row electrodes has either a +Vr voltage level or a −Vr voltage level, K is a power of 2 being L or more, and a column vector element of a K-th orthogonal matrix corresponds to the voltage level. Then, assuming that the total sum of exclusive ORs of corresponding elements between a data vector of ON/OFF display data and a selection voltage vector is i, i is an integer of any one of 0 to L. Voltage values Vi at the level of (L+1) are applied to the column electrodes.
Moreover, JP 11-258575 discloses an example of the MLA drive system, called a BLA3 (Bi-Level Addressing 3) drive system. In this system, three row electrodes are simultaneously selected. A selection voltage for the row electrodes has two voltage levels, that is, +Vr and −Vr. The selection voltage corresponds to column vector elements of three rows and four columns obtained by excluding one row from a fourth orthogonal matrix. The column electrodes are driven by applying two voltage levels: if the total sum of products of corresponding elements between data vector of ON/OFF display data and selection voltage vector is positive, a voltage level corresponding to −1 is applied; if it is negative, a voltage level corresponding to +1 is applied.
Recently, however, the colorization of an LCD panel (liquid crystal display apparatus), which is used as display means in a personal computer, a personal digital assistance, a portable telephone, or the like, has been more and more improved, so that 4K colors, 65K colors and the like have been put into practical use. On the other hand, the attempt of mounting LCD drivers on a single chip is now under development for reducing the cost. However, the area of a display data memory is increased along with the improvement in colorization, resulting in a dilemma that a fine-process with high voltage tolerance should be realized.
For example, the above-mentioned conventional LCD driving systems have the following problems.
More specifically, in the drive system described in JP 6-27904 A, as the number L of row electrodes to be selected at a time is increased, the selection voltage (+Vr, −Vr) can be lowered. However, (L+1) voltage levels are required for the column electrodes. For example, in the case of L=8, L+1=9 voltage levels are required for the column electrodes. As a result, a power source circuit is complicated to disadvantageously increase the size of a driver circuit of the column electrodes.
On the other hand, in the drive system described in JP 11-258575, since the voltage level for the column electrodes has two values, the size of a driver circuit can be reduced. However, the selection voltage cannot be lowered with L=3. In this manner, this drive system is not suitable for a fine-process for its high selection voltage, and therefore is not useful for mounting a driver circuit on one chip. Thus, there is a problem in that the BLA3 drive system is also no more suitable for applications such as a portable telephone.
Furthermore, as described above, the LCD panel is required to display a multi-gradation image with high definition along with its improvement in colorization. At the same time, there is a growing demand for the LCD panel to display full motion pictures.
The known gradation driving systems for multi-gradation display are roughly classified into two types; an FRC (Frame Rate Control) gradation system and a PWM (Pulse Width Modulation) gradation system.
The FRC gradation system uses a plurality of frames to display a single display image. In this system, the frequency of ON/OFF operations is controlled by a voltage to be applied to a liquid crystal device in each frame period so as to express the gradations of a display image.
Also, the PWM gradation system divides one frame period into an ON period and an OFF period so as to express the gradations of a display image. More specifically, the PWM gradation system can be considered as a system for executing the FRC gradation system within one frame.
Moreover, it is necessary to update the display image data of at least 30 frames or more for a second so as to display a motion picture (full motion picture). In order to realize such update, image data should be transferred for each frame, and therefore, it is necessary to overwrite a memory at high speed.
Furthermore, with the increase in number of gradations, the amount of data is also increased. Accordingly, higher speed is required, leading to increased power consumption. Therefore, it is required to restrain the power consumption to be as small as possible so that the power consumption is not increased even if the speed is increased.
Conventionally, as a method of realizing the multi-gradation, for example, JP 11-24637 A discloses the combination of the PWM gradation system and the FRC gradation system for displaying a natural image at 64-gradations or more on a passive matrix liquid crystal display apparatus equipped with a large screen.
In this system, each column voltage is unevenly divided into two sections. In each frame period, the multi-gradation is expressed in the PWM gradation system. The FRC gradation system is combined with the PWM gradation method so that one image is updated in each cycle consisting of a plurality of frames, each frame corresponding to the PWM gradation, thereby constituting the multi-gradation.
For realizing such gradation expression, both column voltage control and phase frame control are employed. The column voltage control is for variably controlling a column voltage in accordance with a series of column voltage sequences which are applied to a predetermined liquid crystal device so as to display a predetermined gradation. More specifically, in the case where a series of column voltage sequences to be applied to a predetermined liquid crystal device or column electrode are all smaller than a pulse width which can be allocated to the column voltages, for example, the column voltages are increased by 5% so as to compensate for the lowered brightness due to a high frequency.
The phase frame control is for controlling a phase so that a mean brightness of a plurality of brightnesses becomes approximately uniform over a plurality of frames in the FRC gradation system.
Furthermore, the system disclosed in the above-described JP 11-24637 A controls the absolute values of the respective column voltages of a series of column voltage sequences to be all the same so as to restrain the generation of splicing, that is, transient brightness offset.
Also, as a conventional apparatus for displaying a motion picture, for example, JP 9-281933 A discloses a liquid crystal display screen (liquid crystal panel) equipped with a static picture display area and a motion picture display area. The switching is performed between static picture data transmitted from a CPU and the like and motion picture data-transmitted from a motion picture controller so as to output the data to the liquid crystal panel.
In this method, the display data (static picture data) from an external data bus is stored into a display memory included therein. The display is performed while switching between an output data bus for sequentially reading out the static data from the display memory and an external data bus carrying display data (motion picture data) from an external motion picture controller, whereby the power consumption is intended to be reduced.
In the method disclosed in the above-mentioned Patent Publication, the gradation display is performed by any one of the FRC system, the PWM system, an AM (amplitude modulation) system, or the combination thereof.
In the gradations obtained by the combination of the PWM system and the FRC system, in particular, each gradation obtained by the PWM for dividing a selection time period of row electrodes (hereinafter, row selection time period) is arranged in a series for each frame to achieve the multi-gradation.
However, in a STN (super twisted nematic) LCD driver compatible to display of a full motion picture obtained by switching at least 30 or more frames on the screen for each second, if the multi-gradation display is intended to be realized by the PWM system alone, the frequency of a column signal becomes higher. As a result, there arises a problem in that the LCD panel cannot respond to such a high frequency. This is mainly caused by a resistance component of transparent electrodes and a capacitance component of liquid crystal between the transparent electrodes.
Moreover, similarly to the method disclosed in the above-mentioned JP 11-24637 A, even if the column-divided PWM is combined with the FRC system to realize the multi-gradation, the amount gradually reduced by the column-divided PWM is gradually increased by the FRC. Therefore, there are problems that a column signal has similarly a higher frequency and that a column selection time period is gradually reduced.
In a conventional duty drive system, a frame response phenomenon is primarily generated in high-speed liquid crystal. Since the high-speed driving is performed in the motion picture display as described above, there is a problem that the contrast is disadvantageously lowered due to the frame response phenomenon. In the MLA drive system, the number of selections for each unit time is increased as compared with that in the duty drive system. However, the same problem arises for a higher frequency.
In the system disclosed in the above-mentioned JP 9-281933 A, for switching between the motion picture data from the exterior and the static picture data present inside, the electric power is consumed in the exterior. There is a problem in that the presence of a plurality of chips increases the cost.
Furthermore, the MLA drive system has a problem in that the brightness unevenness is generated in a horizontal direction. This horizontal brightness unevenness is sometimes referred to as a COM stripe because it is a stripe generated in a row electrode (COMMON electrode) direction.
On the other hand, the column voltage control disclosed in the above-mentioned JP 11-24637 A does not serve as an effective solution for the horizontal brightness unevenness. The column voltage is determined by the result of an MLA calculation (exclusive OR and addition) between ON/OFF display data and an orthogonal function. Therefore, if it is intended to predict a series of column voltage sequences over frames so as to determine whether the column voltage is to be increased or not, the circuit is extremely complicated. Thus, such a solution is not practical.
The invention disclosed in the above-mentioned JP 11-24637 has an object of attenuating a high frequency component of the column voltage sequence by a resistance component of the column electrode and a capacitance component of each liquid crystal. In this case, however, the brightness unevenness appears in a direction of the column electrode (normally, in a longitudinal direction). Therefore, it is believed that such a phenomenon differs from the brightness unevenness (COM stripe) appearing in the direction of the row electrode (normally, in a horizontal direction), which is regarded as a problem in the present invention. Although the reason for occurrence of the horizontal brightness unevenness is not elucidated, it is supposed that such brightness unevenness is caused due to optical response characteristics depending on a pattern of a row electrode voltage and a column electrode voltage applied to liquid crystal in time sequence. Therefore, the above-mentioned related art cannot solve the problem of the horizontal brightness unevenness.